In our U.S. Pat. Nos. 4,267,410 Forster et al, 4,441,202 Tong et al. and Australian Patent Applications Nos. AU-B 41061/78. AU-B 59812/80 and AU-A18194/83, we have described various aspects of a hearing prosthesis system of the above type. In the preferred realizations of this system. the implanted hearing prosthesis comprises three basic components: a receiver/stimulator implanted in the mastoid process, an electrode array implanted in the scala tympani of the inner ear, and the interconnection between the receiver/stimulator and the electrode array. The receiver/stimulator receives a radio frequency signal from an external transmitter demodulates and decodes this signal, and generates electric current which is delivered to the electrodes according to the decoded information.
Investigations of the receiver/stimulator determined that it would be advantageous to have a greater degree of control over the stimulation applied by the array of electrodes. In particular greater control over the distribution of currents along the length of the scala tympani is important because it has been established in psycophysical studies that the timbre of the hearing sensation is closely related to the current distribution.
The preferred form of electrical stimulation utilized in existing hearing prostheses and other neural stimulators is the biphasic constant-current pulse exemplified in FIG. 1. The two phases or halves of this pulse 1 and 2 have equal durations and equal current levels. The direction of current flow in the second phase 2 is opposite to that in the first phase 1. There may be an interphase interval 3 in which no electrode current flows. One important characteristic of this signal is that the amount of charge contained in the second phase of the pulse is exactly equal and opposite to that contained in the first phase. This is because it is essential that the total of all the charge transferred through any electrode by the time of the end of each biphasic current pulse closely approximates zero in order to ensure the safety of long-term electrical stimulation. The duration of the pulse is typically somewhat less than 1 ms.
In the past, hearing prostheses have used one of the following three modes of stimulation: monopolar, common-ground or bipolar stimulation. To produce neural excitation with monopolar stimulation, one or more electrodes implanted in the scala tympani conducts biphasic current pulses such that the total current flows through a single ground electrode located at a remote site. For common-ground stimulation, electrodes interspersed between the active scala tympani electrodes are connected to ground and conduct the current supplied by the active electrodes. In bipolar stimulation, biphasic constant-current pulses are passed between two ala tympani electrodes. A number of such bipolar electrode pairs may be used simultaneously in a hearing prosthesis. Of the three modes of stimulation, it is generally accepted that the bipolar mode provides the greatest degree of control over the distribution of currents in the scala tympani. However, it has been found to be impractical to achieve simultaneous stimulation of two or more electrodes while still achieving a zero charge balance since independent current sources are required for each electrode required to apply simultaneous stimulation and it is impractical to control such current sources to achieve the required degree of charge balance.
It is one specific object of the present invention to ensure that the sum of all the charge transferred through any electrode is as close as fractical to zero at the end of a stimulating current pulse even when several bipolar electrode-pairs are stimulating at discrete sites in the scala tympani substantially simultaneously or quasi-simultaneously. In the present specification, the term "quasi-simultaneously" refers to stimulation which appears to the nerves being stimulated to be simultaneous.
It has been shown in various physiological studies that the amplitude of the gross neural response (i.e. the auditory brainstem response) to an electrical stimulus is determined mainly by the amount of charge delivered to the electrodes during one phase of the biphasic stimulus pulse. The shape of the current waveform producing this charge transfer is relatively unimportant in nerve stimulation; only the amount of charge delivered to the electrodes needs to be accurately controlled, provided that the duration of the electrical stimulus is short (i.e. less than approximately 1 ms). This general characteristic has been verified for Pulsatile current waveforms having duty cycles in the range of 30.degree.-100.degree..
The present invention makes use of this fact in enabling the presentation of electrical stimulation to several scala tympani sites substantially simultaneously or quasi-simultaneously. Each bipolar pair of stimulating electrodes is made to conduct a series of short, discrete elemental constant-current pulses which in total approximates the effect of a single conventional biphasic constant-current pulse of the type shown in FIG. 1.
Thus, in its broadest form the invention provides a means for controlling currents delivered to electrodes for stimulating nerves in prosthetic applications, said current control means being characterized in that current is delivered to each electrode or to each bipolar pair of electrodes in a series of short elemental pulses, each elemental pulse being separated from the next by an interval of zero current which has a longer duration than an elemental pulse. The waveform of the stimulus current comprises a series of said pulses of one polarity followed by an equal number of said pulses of opposite polarity whereby the sum of all the electrical charge transferred through each electrode is approximately zero at the end of a stimulating current waveform. In this way elemental current pulses applied to each electrode or each pair of electrodes which are stimulating simultaneously are preferably delivered cyclically such that elemental pulses delivered to one electrode are interleaved in time with those delivered to any other electrodes, thereby enabling the use of a single current source. Even though the electrodes are not stimulating in a truly simultaneous manner, the capacitance of the system means that it appears to the nerves that they are in fact being stimulated simultaneously so that all of the advantages of simultaneous stimulation are achieved.
The elemental current pulses may be either rectangular constant-current pulses or pulses whose amplitudes vary continuously in time, provided that they are interspersed with zero-current intervals as described.
In one specific form, the present invention provides in a receiver/stimulator for a hearing prosthesis comprising means for receiving a transmitted data signal, means for decoding the received signal, means for generating and delivering several independent current signals to a selected subset of stimulating electrodes, configured as a number of bipolar pairs, according to the decoded information, the improvement comprising means for controlling said current signals to cause said bipolar pairs to conduct a series of short elemental constant-current pulses each separated by zero-current intervals longer than each elemental pulse. said series of elemental pulses being divided into two halves having opposite current direction whereby the sum of the charge transferred through each of the selected bipolar electrode pairs is exactly zero when the delivery of the current signals is complete.